It will be understood that when reference is made in the remainder of this application to CVD or epitaxial deposition processes, these are merely prime examples of the range of thermal processes to which the teachings of the present invention are applicable.
The commercial production of semiconductor devices has in recent times been placed under increasing pressure to reduce the production cost. This in turn has required new measures to increase the efficiency of epitaxial processing methods so that they yield higher throughput of acceptable devices at a lower cost per device. One important recent development in this regard is a compact, double-dome reactor which achieves increased processing speed and reduced consumption of the gases used in the epitaxial reaction.
Such an apparatus is fully detailed by Anderson et al. in U.S. Pat. No. 5,108,792, incorporated herein by reference and hereinafter referred to as the '792 patent. The central concepts of this double-dome reactor system may be summarized as follows: (1) substrate support on a thin, low-mass susceptor for rapid heating and cooling; (2) substrate and susceptor enclosure in a compact housing having a pair of transparent domes, each dome covering one face of the susceptor-substrate combination; (3) conduits for reactant gases to enter and exit the housing; and (4) a pair of radiant heaters arranged to project thermal radiation through each of the transparent domes to uniformly heat each face of the susceptor-substrate combination.
While the reactor system described in the '792 patent has proven very efficient in reducing processing cost and increasing throughput, work has continued on further improvements in these regards. The radiant heaters shown and described in the '792 patent use a pair of concentric arrays of heater lamps in a simple cylindrical reflector housing, one lamp array near the outer periphery and the other nearer the center of the cylindrically symmetric arrangement. The two arrays use different types of lamps having different radiation patterns, as shown in the drawing of the '792 patent.
Such an arrangement achieves good radial uniformity of thermal radiation from the center to the edge of the substrate, while rotation of the substrate about the axis of cylindrical symmetry effectively cancels any azimuthal non-uniformities of radiation. However, achieving similar radial uniformity of thermal radiation without requiring two concentric arrays of heater lamps of two different types is very desirable, since the cost of the heaters could be reduced.
A compact double-dome reactor that achieves radial uniformity without requiring two concentric arrays of heater lamps of two different types is shown by Anderson et al. in U.S. Pat. No. 5,179,677, incorporated herein by reference and hereinafter referred to as the '677 patent. The apparatus of the '677 patent uses a plurality of linear, tubular heater lamps arranged in a concentric radial array about an axis along which thermal radiations are directed toward the substrate. Some of the lamps are provided with focussing reflectors that cause thermal radiation to be directed in parallel paths emanating from the focusing reflectors toward the substrate--the result being that greater thermal radiation is directed at the center of the substrate. The remainder of the lamps are provided with dispersive reflectors that cause thermal radiation to be dispersed in a hemispherical radiation pattern. A peripheral reflector circumferentially surrounds the lamps and their associated reflectors so as to intercept some of the dispersed radiation. The peripheral reflector redirects the radiation it intercepts into a radiation pattern that is more intense at the periphery of the substrate than at the center. The result is a pattern of radiation from the lamps with focusing reflectors that is balanced across the substrate. The reactor of the '677 patent achieves improved radial uniformity of thermal radiation from the center to the edge of the substrate without using two concentric arrays of different type heater lamps, while rotation of the substrate about the axis of cylindrical symmetry effectively cancels any azimuthal non-uniformities of radiation. It is known to use a temperature sensor directed at the bottom of the susceptor of the '677 patent and, through a servo loop, to control both lamp arrays.
However, beyond the teachings of the '677 and '792 patents, selective control of patterns of radiation directed to the upper and lower surfaces of the susceptor-substrate combination to produce a controlled uniform or predetermined axial heating of and temperature profile for the substrate is very desirable for several reasons. First, during temperature ramp up, which occurs when the substrate is heated from, e.g., room temperature to CVD processing temperatures, if heat is applied equally to upper and lower surfaces of the susceptor-substrate combination, the substrate will heat much more rapidly on its upper surface, i.e., the surface exposed directly to thermal radiation, than on its lower surface, i.e., the surface that is in contact with the susceptor. This is because the susceptor blocks direct radiation to the lower surface of the substrate and has a sufficient mass, a sufficient lack of thermal conductivity and a sufficiently small thermal heat transfer coefficient, to delay the application and reduce the amount of heat applied to the lower surface of the substrate from the lower heater lamps. In addition, a center portion of the upper surface of the susceptor will be blocked by the substrate from receiving thermal radiation from the upper heater lamps. As a result, the susceptor will heat much more slowly in its blocked center portion, than in its unblocked peripheral portion. Hence, the substrate will initially be hotter than that portion of the susceptor that is adjacent to and blocked by the substrate. Then as the susceptor-substrate combination heats to the CVD processing temperature, the substrate tends to follow the temperature of the susceptor, and, as a result, the substrate will develop an undesired radial temperature gradient or profile wherein the center of the substrate will tend to be cooler than its periphery. Since a uniform steady state temperature profile is generally desired for substrate processing, the processing of the substrate must wait for a cooling and equalizing of the temperature within the substrate, which can take several minutes to achieve. Thus, a selective control of the radiation patterns applied to the upper and lower surfaces of the susceptor-substrate combination which maintains uniform temperature ramping and a uniform temperature profile within the substrate ramp up is desired. The axial heat transfer tends to produce a small axial gradient within the wafer and within the susceptor, with a larger temperature difference between the wafer and susceptor due to thermal contact resistance at the interface.
A second problem that could be addressed by a system capable of achieving axial and radial uniformity of temperature within a substrate is that of gas flow effect. In the process chamber used for CVD processes, the resulting deposition layer uniformity may be affected by the flow of a deposition gas stream. Certain gas species, such as trichlorosilane may be depleted due to deposition at the surface or dilution by other flowing gasses. Advantageously, such non-uniformities may be compensated for by producing a non-uniform temperature profile (or gradient), both radially and axially, that compensates for the flow effect.
Third, the problem of stress induced slip deformation of a substrate can be caused by non-uniform temperature profiles in the substrate during heating or cooling.
A substrate that is hotter at its periphery than at its center will be subjected to a compressive stress at its outer edge. Similarly, a substrate that is hotter at its center than at its periphery will be subjected to compressive stress at its center. A system capable of achieving a controlled radial temperature profile in a substrate may control such stress induced slip.
Finally, it is known that atoms can be transferred by a temperature gradient if the temperature gradient is selectively controlled. This phenomenon can be utilized to cause a coating to be transferred to the substrate, or to prevent the transfer of atoms between the susceptor and the substrate. Again, a system capable of selectively achieving predetermined axial temperature gradients in a substrate/susceptor combination may be desired to control the transfer of atoms between a susceptor and a substrate.
Thus, improvements are needed in heater systems used in thermal processing, particularly in the rapid thermal processing of semiconductor substrates during chemical vapor deposition (CVD) processes, including epitaxial reaction processes. The present invention advantageously addresses the above and other needs.